1. Field of the Invention
The present arrangement relates to communication cables. More particularly, the present arrangement relates to data communication cables using modified insulation.
2. Description of the Related Art
In the communication industry, one type of a common communication cable is the LAN (Local Area Network) cable, formed from four pairs of conductors. The conductor pairs are made from two wires twisted around one another, commonly referred to as a twisted pair. Typical high speed communication cables may include a number of shielded or unshielded twisted pairs enclosed by an outer jacket.
One problem that typically confronts the construction of such cables is signal interference or crosstalk that can occur between twisted pairs within the cable as well as with interference from other signal sources outside the cable, in particular with unshielded twisted pairs running in adjacent cables. In order to reduce the incidences of cross talk, the twisted pairs in unshielded data communication cables have different twist rates from one another so that a typical four pair LAN cable will have 4 pairs each with a different twist rate.
However, due to the different twist rates for addressing crosstalk, another cable construction obstacle arises referred to as skew. For example, for any given length of cable, the same signal sent along two adjacent twisted pairs with different twist rates will reach the end of the cable at different times. This occurs because the twisting of one pair at a shorter lay length (higher twist rate) than another pair within the same cable will necessarily result in the physical conductor path in the shorter lay length pair being longer than the conductor path of the pair(s) with the longer lay length (slow rate of twist). This resultant time difference is known as skew.
For example, in a 1000′ cable, each of the twisted pairs would exceed 1,000 feet in length because they are twisted. Assuming normal sized copper conductors/insulation for LAN cables, the typical lengths for the pairs would result in approximately 1,010 feet of wire needed for each wire in the fastest (longest lay length) pair, approximately 1,030 feet of wire needed for each wire in the slowest (shortest lay length) pair, with some amount in between needed for the other pairs.
As a result, a signal travelling down the longest lay length pair would arrive about 2% sooner than a signal travelling down the shortest lay length pair. According to most testing standards, there is a requirement that for a 100 meter length of cable 10, the time difference it takes for a signal to travel from one end of cable 10 to the other, between any two pairs cannot exceed 45 nanoseconds.
The property of skew and the associated signal/time difference is not influenced only by the physical length of the conductors in the various pairs. The insulation used on the pairs also affects the speed of signal propagation due to dielectric characteristics created by the insulation layer(s). This effect is a result of the communication signal passing in part through the insulation on the conductor pairs, slowing the propagation rates. Thus, in the longer (shorter lay length) pairs, the dielectric coupling of the signal to the insulation slows the propagation rates.
Moreover, each polymer used for insulation has its own dielectric constant. Certain polymers have low dielectric constants with a corresponding lesser effect on the signal speed. An example of such a polymer is FEP (Fluorinated Ethylene Propylene Copolymer). Other polymers such as Polypropylene have higher dielectric constants and thus exhibit a greater negative effect on the signal speed. This further exacerbates the skew problem. Many LAN cables employ two or more different types of insulation on the different pairs within the same cable.
One way the prior art has addressed the problem of skew is to increase the relative signal propagation velocity in the slower pairs by foaming the insulation used on those pairs. By foaming the insulation, the dielectric constant is reduced, thus allowing the signal in the slow pairs (pairs with shorter lay length) to be faster relative to the faster pair (pair with the longest lay length) reducing the overall signal velocity difference in the cable pairs and thus reducing skew.
However, the foaming process has a number of disadvantages; it is expensive, causes reduced manufacturing line speeds (slow extrusion), is difficult to control and ultimately yields high scrap rates. In addition, foamed insulation is easier to crush and thus may lead to the cables/pairs failing the necessary crush resistance testing. In fact, the foamed insulation may even overly compress/crush during twining (of the conductors into pairs). As a result, the insulation on the foamed pairs must be oversized to compensate. This increases the overall diameter of the cable which creates problems for the end user since smaller diameter cables are usually preferred.
One manner for overcoming these drawbacks is to manipulate the electrical properties of the conductor insulation in the twisted pairs by compounding additives into the polymer and extruding these compositions onto wire as a primary coating of plenum cable twisted pairs to obtain regularized electrical performance between the pairs in a cable. In this respect, instead of speeding up signal propagation in the slow pairs of a cable to reduce skew, as is the case in the prior art, the introduction of additives into the insulation in the fast pairs (longest lay length) reduces the signal propagation speed to even the propagation speed among the four pairs in a typical LAN cable thus reducing skew.
In this context, different additives had been used within the insulation of the fast pair, including but not limited to glass beads, talc, zinc oxide and calcium fluoride. Although these additives may exhibit certain advantageous electrical properties they otherwise negatively affect the processability (extrusion quality/speed etc. . . . ) of the insulation as well as having negative effects on the dissipation factor (the ratio of the power loss in a dielectric material to the total power transmitted through the dielectric.)